Photoactive devices are semiconductor devices that employ semiconductor material to convert electromagnetic radiation into electrical energy or to convert electrical energy into electromagnetic radiation. Photoactive devices include, for example, photovoltaic cells, light-emitting diodes, and laser diodes.
Photovoltaic cells (also referred to in the art as “solar cells” or “photoelectric cells”) are used to convert energy from light (e.g., sunlight) into electricity. Photovoltaic cells generally include one or more pn junctions, and can be manufactured using conventional semiconductor materials, such as silicon, as well as III-V semiconductor materials. Photons from impinging electromagnetic radiation (e.g., light) are absorbed by the semiconductor material proximate the pn junction, resulting in the generation of electron-hole pairs. The electrons and holes generated by the impinging radiation are driven in opposite directions by a built-in electric field across the pn junction, resulting in a voltage between the n region and the p region on opposing sides of the pn junction. This voltage may be used to produce electricity. Defects in the crystal lattices of the semiconductor materials at the pn junctions provide locations at which electrons and holes previously generated by absorption of radiation can recombine, thereby reducing the efficiency by which the radiation is converted into electricity by the photovoltaic cells.
The photons of the electromagnetic radiation that impinge on a photovoltaic cell must have sufficient energy to overcome the bandgap energy of the semiconductor material to generate an electron-hole pair. Thus, the efficiency of the photovoltaic cell is dependent upon the percentage of the impinging photons that have an energy corresponding to the bandgap energy of the semiconductor material. Stated another way, the efficiency of the photovoltaic cell is at least partially dependent upon the relationship between the wavelength or wavelengths of the radiation impinging on the photovoltaic cell and the bandgap energy of the semiconductor material. Sunlight is emitted over a range of wavelengths. As a result, photovoltaic cells have been developed that include more than one pn junction, wherein each pn junction comprises semiconductor material having a different bandgap energy so as to capture light at different wavelengths and increase the efficiencies of the photovoltaic cells. Such photovoltaic cells are referred to as “multi-junction” or “MJ” photovoltaic cells.
Thus, the efficiency of a multi junction photovoltaic cell may be increased by selecting the semiconductor materials at the pn junctions to have band-gap energies that are aligned with the wavelengths of light corresponding to the wavelengths of highest intensity in the light to be absorbed by the photovoltaic cells, and by decreasing the concentration of defects in the crystal lattices of the semiconductor materials at the pn junctions. One way to decrease the concentration of defects in the crystal lattices of the semiconductor materials is to employ semiconductor materials that have lattice constants and coefficients of thermal expansion that are closely matched with one another.
It has been proposed to employ the dilute nitride III-V semiconductor material Ga1-yInyNxAs1-x, wherein y is about 0.08 and x is about 0.028, in one pn junction of a multi junction photovoltaic cell. Such a dilute nitride III-V semiconductor material may exhibit a bandgap energy of from about 1.0 eV to about 1.1 eV.
Such dilute nitride III-V semiconductor materials have proven difficult to fabricate, at least on a commercial scale. These difficulties are partly due to the disparities in the atomic radii of the various elements of the material, which range from about 0.75 Angstroms to about 1.62 Angstroms. Examples of methods that have been used to fabricate GaInNAs are disclosed in, for example, Dimroth et al., Comparison of Dilute Nitride Growth on a Single-and 8×4-inch Multiwafer MOVPE System for Solar Cell Applications, JOURNAL OF CRYSTAL GROWTH 272 (2004) 726-731, and in Chalker et al., The Microstructural Influence of Nitrogen Incorporation in Dilute Nitride Semiconductors, JOURNAL OF PHYSICS: CONDENSED MATTER 16 (2004) S3161-S3170, each of which is incorporated herein in its entirety by this reference.